Conventionally, there has been proposed a variety of composite ceramic bodies formed of a first bonding body comprising a ceramic, and a second bonding body composed of other ceramics which is bonded to the first bonding body with an intermediate layer in between.
As the intermediate layer, aluminium, silver, gold, platinum, or alloys of these metals have been used. By coating a metallic brazing filler metal to the above-mentioned bonding bodies and/or the intermediate layer, a stacked body is formed and then heat-treated to obtain a composite ceramic body (refer to patent literature 1).
As a composite ceramic body using a bonding body composed of alumina ceramics, there has been proposed a composite ceramic body in which bonding bodies are bonded to each other with an intermediate layer in between. The bonding bodies are composed of alumina ceramics containing a YAG composition and having an alumina-YAG graded layer in which the ratio of the YAG composition is increased from the inside to the surface. In a method of manufacturing the composite ceramic body, granuled power composed of alumina and a predetermined amount of a yttrium compound are formed to obtain formed bodies. The formed bodies are sintered at 1600 to 1850° C. with their respective bonding parts superposed, resulting in the composite ceramic body. Alternatively, the above-mentioned formed bodies are calcinated at 800 to 1300° C., to obtain calcinated bodies. The calcinated bodies are sintered at 1600 to 1850° C. with their respective bonding parts superposed, resulting in the composite ceramic body (refer to patent literature 2).
As a composite ceramic body of ceramic sintered bodies having substantially the same composition, there has been proposed a ceramic sintered body in which a bonding layer contains 90 weight % of the component composition of the ceramic sintered bodies. The composite ceramic body has a thickness of 50 to 500 μm (refer to patent literature 3).
There has also been proposed that the ratio of the mean crystal grain size of a ceramic sintered body and the mean crystal grain size of a ceramic sintered body constituting a bonding layer is 0.5 to 2.0. There are the following descriptions. In a method of manufacturing these composite ceramic bodies, ceramic formed bodies prior to sintering, having substantially the same ceramic component composition, are bonded with a ceramic-based slurry adhesive, followed by sintering. Here, it is suitable to use the ceramic-based slurry adhesive in which the component composition of ceramic particles in the ceramic-based slurry contains 90 weight % or more of ceramic composition constituting the ceramic formed bodies, having a mean particle size of 1.0 μm or less, and has a water content of 5 to 25 weight %.
There has also been proposed a composite ceramic body formed by bonding bodies composed of low thermal expansion ceramics with an intermediate layer in between. The intermediate layer is composed of low thermal expansion ceramics having a lower melting temperature than the bonding bodies. In this composite ceramic body, only the intermediate layer can be melted during the time of bonding, thereby to bond a plurality of bonding bodies to each other. Further, since the intermediate layer is the low thermal expansion ceramics, the stress remaining in bonding surfaces is small and the rigidity of the bonding surfaces is high. Therefore, the rigidity of the entire composite ceramic body can be increased, and the strength of the bonding surfaces themselves can be increased (refer to patent literature 4).
In a method of bonding synthetic corundums, it has been proposed to obtain bonding by superposing the polished bonding surfaces of two synthetic corundums to bring one end part into an adhesion state, and bonding in this state at a temperature not above the melting point of the synthetic corundums (1100 to 1800° C.). It is described that higher bonding strength is attainable by controlling the flatness of the bonding surfaces of the above-mentioned synthetic corundums to a range of 1/2 to 1/6 of the wavelength λ of red light (refer to patent literature 5).
As a method of bonding silicon-oxide-based members, there has been proposed a method of bonding two silicon-oxide-based members by exposing them to hydrofluoric acid gas and then bringing them into contact with each other. There are the following descriptions. In this bonding method, the two silicon-oxide-based members are placed at opposite positions, and hydrofluoric acid gas is filled in between, so that the hydrofluoric acid gas is adsorbed on the respective surfaces of the silicon-oxide-based members, and the bonding between atoms are cut, resulting in chemically active. That is, hydrofluoric acid gas is the vapor of hydrofluoric acid generated when hydrofluoric acid (HF) is dissolved in water (H2O), and hydrofluoric acid is dissolved in water and then ionized. Subsequently, hydrofluoric acid gas is adsorbed on the surfaces of the silicon-oxide-based members, and the siloxane bonding of the silicon-oxide-based members is cut, resulting in chemically active. The silicon-oxide-based members are then brought into contact with each other, so that two chemically active surfaces are connected and bonded to each other (refer to patent literature 6).
In the above-mentioned composite ceramic bodies in patent literatures 1 to 4, there is the following problem. That is, when the intermediate layer is formed in advance between the bonding bodies, and brought into contact with each other and then heat-treated, variations at the time of forming can be reflected thereby to cause variations in the thickness of the intermediate layer.
On the other hand, in the case of bonding a mono-crystalline such as sapphire and the ceramic body, as in patent literatures 6 and 7, there is the problem that the reaction of bonding interface is induced thereby to impair mechanical and electrical characteristics possessed by nature.
In the composite ceramic body in which the adhesive is disposed in the intermediate layer, because high temperature processing is required in use, reliability may be a problem when this is used in a manufacturing process and high temperature atmosphere. The composite ceramic body by means of molten deposition has the problem that dimensional accuracy is liable to be lowered because heat-treated fused powder is bonded by high-pressure spraying in the manufacturing process. Additionally, in the bonding using electrical energy, the bonding between chrome or silicon and glass is attained by the application of voltage. The bonding with crystals cannot be attained without the presence of a metal film or the like, it is limited to a temperature range where a material to be bonded cannot be dissolved. Hence, an upper limit of the operating temperature range is imposed.
With this in view, a composite ceramic body has been proposed in which a main surface of a bonding body is processed for direct bonding, without using any intermediate layer composed of adhesive or the like. For example, as an electronic part used in a device, a magnetic substrate and a retention substrate which is brought into a direct bonding with the magnetic substrate by at least one of hydrogen bond and covalent bond have been proposed (refer to patent literature 7). There are the following descriptions. The term “direct bonding” is defined as bonding which can be formed on the substrate by hydrogen bond or covalent bond, without using other material such as adhesive. The formation of the direct bonding is obtained as follows. Firstly, the surface of at least one of the magnetic substrate and the retention substrate is subject to hydrophilization process in order to introduce hydroxyl groups into the substrate surface. Then, when the surfaces having sufficient hydroxyl groups are superposed, the two substrates can be bonded by hydrogen bond between hydroxyl groups or through adsorbed water molecules. By heat treatment of the bonded substrates, the water molecules and hydrogen can be released from the bonding interface, and hydrogen bond can be shifted to covalent bond, thereby increasing bonding strength. The heat treatment temperature in the direct bonding is set to 200 to 800° C.
A similar bonding body as a crystal element has also been proposed. That is, crystal wafers are mirror-polished and bonding surfaces are subject to hydrophilization process (OH group conversion). Further, after the crystal wafers are brought into contact with each other, the OH groups on the interface are removed by heat treatment, so that bonding can be obtained by Si—O—Si bonding (interatomic bonding) (refer to patent literature 8).
In the above-mentioned direct bonding, as shown in FIGS. 9(a) to 9(c), when the surface of a first bonding body 21 and the surface of a second bonding body 22 are subject to hydrophilization process, a large number of water molecules and OH groups can be adsorbed on the surface of the first bonding body 21 and the surface of the second bonding body 22.
When in this state, the first bonding body 21 and the second bonding body 22 are brought into contact with each other, the initial bonding between the first and second bonding bodies 21 and 22 by hydrogen bond through water molecules, OH group, or the like can be formed (FIG. 9(a)). Subsequently, when these are heat-treated, dehydration or hydrogen release occurs gradually from the bonding interface, thereby enhancing the bonding (FIG. 9(b)). The increased strength may result from the fact that the dominant bonding of direct bonding is shifted from hydrogen bond through water molecules, to hydrogen bond between OH groups without through water molecules (FIG. 9(c)). This state can be often observed when heat treatment is carried out at temperature of 200 to 500° C. For attaining strong bonding, heat treatment temperature is set in consideration of the coefficient of thermal expansion and the shape (dimension) of materials to be bonded. Specifically, the heat treatment is carried out at temperatures of 200 to 800° C.    Patent literature 1: Japanese Unexamined Patent Publication No. 2000-143363    Patent literature 2: Japanese Unexamined Patent Publication No. 2003-48783    Patent literature 3: Japanese Unexamined Patent Publication No. 2003-128473    Patent literature 4: Japanese Unexamined Patent Publication No. 2004-59402    Patent literature 5: the pamphlet of WO 00/37720    Patent literature 6: Japanese Unexamined Patent Publication No. 2002-97040    Patent literature 7: Japanese Unexamined Patent Publication No. 7-220923    Patent literature 8: Japanese Unexamined Patent Publication No. 2001-122697